Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

From: Andy Lutomirski
Date: Fri Jun 07 2019 - 14:34:10 EST

> On Jun 7, 2019, at 10:59 AM, Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>> On 6/7/19 10:43 AM, Peter Zijlstra wrote:
>> I've no idea what the kernel should do; since you failed to answer the
>> question what happens when you point this to garbage.
>> Does it then fault or what?
> Yeah, I think you'll fault with a rather mysterious CR2 value since
> you'll go look at the instruction that faulted and not see any
> references to the CR2 value.
> I think this new MSR probably needs to get included in oops output when
> CET is enabled.

This shouldnât be able to OOPS because it only happens at CPL 3, right? We should put it into core dumps, though.

> Why don't we require that a VMA be in place for the entire bitmap?
> Don't we need a "get" prctl function too in case something like a JIT is
> running and needs to find the location of this bitmap to set bits itself?
> Or, do we just go whole-hog and have the kernel manage the bitmap
> itself. Our interface here could be:
> prctl(PR_MARK_CODE_AS_LEGACY, start, size);
> and then have the kernel allocate and set the bitmap for those code
> locations.

Given that the format depends on the VA size, this might be a good idea. I bet we can reuse the special mapping infrastructure for this â the VMA could
be a MAP_PRIVATE special mapping named [cet_legacy_bitmap] or similar, and we can even make special rules to core dump it intelligently if needed. And we can make mremap() on it work correctly if anyone (CRIU?) cares.

Hmm. Can we be creative and skip populating it with zeros? The CPU should only ever touch a page if we miss an ENDBR on it, so, in normal operation, we donât need anything to be there. We could try to prevent anyone from *reading* it outside of ENDBR tracking if we want to avoid people accidentally wasting lots of memory by forcing it to be fully populated when the read it.

The one downside is this forces it to be per-mm, but that seems like a generally reasonable model anyway.

This also gives us an excellent opportunity to make it read-only as seen from userspace to prevent exploits from just poking it full of ones before redirecting execution.